1. Field of the Invention
This invention relates to carbonaceous diaphragms which exhibit electroacoustic characteristics suitable for use in speakers, microphones and the like and a method for making such diaphragms.
2. Description of the Prior Art
A recent growing tendency toward digitalization of acoustic apparatus requires very high performance of diaphragms for use in speakers or the like. The diaphragm adapted for this purpose should have a small degree of deformation when applied with an external force with a small degree of sound distortion and is able to reproduce a clear sound whose range is wide. To this end, it is necessary that the diaphragm be light in weight and have a good modulus of elasticity and good rigidity. More particularly, the diaphragm should have (1) a large Young's modulus (E), (2) a small density (.rho.), (3) a large sound velocity (transmission velocity V of sound wave), (4) an appropriate value of internal loss (tan.delta.). It will be noted that the values of V. E and .rho. have the relationship that V=.sqroot.E/.rho..
Aside from the above physical properties, it is important that the fabrication be easy and that the diaphragm be stable against heat and humidity.
The currently employed materials for the diaphragm are, for example, paper, plastic resins, aluminium, titanium, magnesium, berylium, boron, silica and the like. These materials or metals have been employed singly or in combination with glass fibers, carbon fibers and the like. Alternatively, some of them have been used in the form of metal alloys. However, paper or plastic resins are not satisfactory for use as a diaphragm with respect to the physical and acoustic characteristics such as the Young's modulus, density and sound velocity. In particular, the frequency characteristic in a high frequency range is very poor, which makes it difficult to obtain a clear sound when these materials are applied as a diaphragm of a tweeter. On the other hand, aluminium, magnesium and titanium are excellent in sound velocity but has so small an internal loss of the vibrations that a high-frequency resonance phenomenon undesirably appears. Thus, these metals are not satisfactory for use as a diaphragm for high-frequency service. Moreover, boron and berylium exhibit better physical properties than those of the above-mentioned materials or metals and can reproduce a sound of good quality when applied as a diaphragm. However, boron or berylium is very expensive and has very poor workability.
On the other hand, diaphragms made of carbon or carbonaceous materials have been recently developed in order to overcome the drawbacks involved in the above-described materials. As is known in the art, graphite has a number of good physical properties which are favorable when graphite is applied as a diaphragm. Several techniques of making diaphrams from graphite or other carbonaceous materials have been proposed including (1) a technique wherein graphite powder is dispersed in a polymer resin or resins, (2) a technique using a polymer sheet which has been carbonized and graphitized, and (3) a technique which makes use of a graphite/carbon combination which is obtained by firing a sheet of graphite powder and a polymer resin or resins.
A typical example of the diaphragm obtained by (1) is one which is made of a dispersion of graphite powder in polyvinyl chloride resin matrix. This diaphragm is readily influenced by humidity and temperature and its vibration characteristic considerably deteriorates at temperatures over 30.degree. C.
With the technique (2), several types of polymer films have been investigated but initially expected characteristics could not be obtained because plastic films used are hard to graphitize. For example, the resins including epoxy resins, phenolic resins, furfuryl alcohol resins have been used for this purpose. These plastic resins exhibit a low rate of graphitization and are shrunk to an appreciable extent when thermally treated, so that defects such as deformation, crackings and the like are often produced. This technique does not ensure fabrication of a diaphragm of graphite or a carbonaceous material having good characteristics under well-controlled quality control.
The technique (3) includes a method wherein a liquid component of a pitch obtained by cracking of crude oil is mixed with graphite powder and the mixture is thermally treated for carbonization, and a method wherein a monomer or oligomer capable of yielding a thermosetting resin is used as a binder for graphite powder and a thermoplastic resin having functional groups capable of thermal decomposition and crosslinkage under heating conditions are mixed with graphite powder as a binder, followed by thermal carbonization. These methods have been developed in order to increase a yield of graphite or carbon and to prevent shrinkage or deformation when thermally treated. A diaphragm obtained from th resultant graphite exhibits good electroacoustic characteristics.
However, the methods of the technique (3) require complicated fabrication procedures which are inconvenient for industrial mass production. In order to industrially obtain the pitch and liquid component by cracking of crude oil in the former method, a very complicated procedure of thermal treatment at high temperatures and fractional solvent extraction is necessary. The latter method requires a high technic wherein graphite powder and a binder are sufficiently kneaded by the use of a kneader operating under high shear force conditions. Subsequently, cleft graphite crystals and the binder resin are strongly dispersed to impart affinity for each other by mechanochemical reaction thereby causing the crystal planes of the graphite to be oriented along the direction of the sheet plane. Although the diaphragm obtained using the resultant combination has very excellent characteristics, those characteristics are slightly inferior to those of a berylium diaphragm which is believed to have the highest characteristics attained among existing diaphragms. In addition, the modulus of elasticity of the combination is significantly poorer than the theoretical value of 1020 GPa. of a graphite single crystal.